Local area networks use a network cable or other network media to link nodes (e.g., workstations, routers and switches) to the network. Each local area network architecture uses a media access control (MAC) enabling network interface device at each network node to share access to the media.
Physical (PHY) layer devices are configured for translating digital packet data received from a MAC across a standardized interface, e.g., a Media Independent Interface (MII), into an analog signal for transmission on the network medium, and reception of analog signals transmitted from a remote node via the network medium. An example is the 100BASE-TX Ethernet (IEEE Standard 802.3u) receiver, configured for receiving a three-level MLT-3 encoded analog signal (hereafter referred to as MLT3 signal) at a 125 Mb/s data rate. For example, FIGS. 1A, 1B and 1C are diagrams illustrating an original NRZI-encoded digital signal for transmission on the media, an MLT3 signal generated at the transmitting PHY layer for transmission on the network medium, and an MLT3 signal having been transmitted by the network medium and received by a PHY transceiver at a destination node. As shown in FIG. 1A, the original BI-level digital signal, encoded as an NRZI signal, is encoded into the three-level MLT3 signal of FIG. 1B before transmitting the digital information to the receiving station.
On the receiving side, the MLT3 signal of FIG. 1B encounters transmission loss, for example high frequency attenuation caused by the cable, resulting in the received MLT3 signal of FIG. 1C. Since the amount of signal attenuation caused by the cable is directly proportional to the cable length, the recovery of the MLT3 (3 level NRZI) signal depends on correct measurement of the length of the cable.
One manner of determining cable length is to measure the amplitude of the incoming signal. However, in the 100 BASE-TX standard, the incoming MLT3 signal may consist of a variety of pulses ranging from 9 ns to approximately 500 ns in length. Consequently, the frequency content of the data varies. Since the cable attenuates high frequencies more than low frequencies, the amount of attenuation of the incoming pulses will vary based on their length (frequency). For example, a 500 ns pulse will have the same peak amplitude after being transmitted on 100 m of cable as that of a 8 ns pulse after being transmitted on 10 m of cable. Therefore, in order to determine cable length by measuring pulse amplitude, pulse length must also be measured and then compared to the amplitude. Such dual measurement system is quite complicated.
Another manner of determining cable length employs signal edge rate. This method involves measuring the rise or fall time of the signal in order to determine the length of the cable the signal has traversed. The longer the cable, the slower the rise/fall time of the signal will be. The advantage of this method is that it is independent of the frequency of the incoming data; only the rise or fall time of the signal matters. However, the disadvantage of this method is that it involves the measurement of very small time quantities; e.g., signal rise time could be as short as 3 ns, which requires a very precise timer in order to detect the difference between rise times after various cable lengths. Another disadvantage is that this method requires some amplitude measurement also since a rise time is a measurement of the change of signal amplitude vs. time. Finally, noise on a specific signal edge may change the measurement result, so several measurements need to be made and averaged in order to obtain the correct cable length.